This invention relates to improvements in sheet clamping apparatus for high speed sheet handling drums and, more particularly, it concerns a light-weight arrangement of axially orientated, drum-carried clamping bars by which the leading and trailing edges of sheet media may be secured firmly along the full length of each such edge by any of several forms of actuating mechanisms supported independently of the drum.
Laser printing technology has developed to a stage where high resolution continuous tone images of a quality commensurate with or superior to those now provided by chemical processing of photographic sheet materials may be produced. In general, laser printing requires modulated laser light to be focused to a point in a sheet medium contained focal plane which must be held to within a fraction of a micrometer to optimize the resolution attainable in the laser modulated light. The focused laser light must then traverse the entire area of the image to be formed in a series of contiguous tracks or lines. This latter operation is ideally formed by supporting the sheet medium about the cylindrical periphery of a rotating drum and traversing the axial dimension of the medium with the focus point of the modulated laser light. Because of the large number of tracks which must be traversed, the time required to record an image on the sheet medium in this manner is almost entirely a function of the speed at which the medium is moved by the drum.
To be cost effective in relation to more conventional techniques for forming continuous images on sheet media, the laser printing operation is preferably carried out at drum speeds on the order of 1500-6000 rpm. At such speeds, centrifugal forces acting on the sheet medium tend to separate the medium radially from the peripheral surface of the drum to a degree giving rise to displacement of the medium as well as tangential forces acting upon the clamped margins of the medium. It is important, therefore, that the mechanism used to secure the medium to the drum be capable of resisting such forces. In addition, dynamic balance of the drum at such high speeds requires that the drum carried clamping mechanism be embodied in an assembly of components which is maintained balanced during drum rotation. At the same time, the clamping mechanism must be capable of resisting movement and/or deflection, or compensate therefor, under the centrifugal forces incurred.
While the problems of retaining a sheet against the centrifugal forces developed by carrying the sheet on a high speed rotating drum have been addressed in such relatively low tolerance applications as sanding drums and the like, the clamping arrangements employed in the prior art are relatively complicated and incapable of maintaining the sheet medium on a drum in a predictable relationship required by the optics of a laser printing system. In a commonly assigned copending application Ser. No. 034,665, filed Apr. 6, 1987, the problem associated with retaining sheet clamping bars against displacement under centrifugal force during drum rotation at speeds on the order of 1600 rpm are addressed. In the disclosure of this application, pivotal centrifugally actuated members are employed to draw the central portion of the sheet clamping bars inwardly in opposition to the centrifugal force. The approach represented by the disclosure of the afore-mentioned co-pending application is satisfactory for the drum revolution speeds contemplated by that disclosure. However, the relatively moveable organization of components supported by the drum gives rise to problems associated with balancing the drum at speeds in excess of 1.600 rpm, possibly up to 6000 rpm.
In light of the current state of the art relative to drum clamping arrangements for retaining sheet media about the periphery of the drum and the commercial potential of laser technology applications in the production of continuous tone images, there is need for an improved apparatus for securing a sheet medium to a high speed drum without concern for displacement of the sheet medium or the clamps from a predictably precise radial as well as tangential position on the drum.